As a method of controlling a torque of an alternating current (AC) electric motor, there are known an electric current control method and a voltage control method. As the electric current control method, for example, there is known a pulse width modulation (PWM) control in which an electric current is controlled based on a vector control. As the voltage control method, there is known a rectangular wave control in which rotation of the AC electric motor is driven by applying a rectangular wave voltage. In addition, as a control for improving an output power in a so-called flux weakening area, there is known a voltage phase control in which a torque of the AC electric motor is controlled by controlling a voltage phase depending on a difference between a torque command value and an actual torque in a rectangular wave voltage control. Furthermore, there is also known a configuration capable of switching a control method depending on a situation by providing such control methods as switchable control modes.
However, if a steady-state torque is different between the control modes, a so-called torque discontinuity is generated when the control mode switches, and this may make a driver feel uncomfortable. In JP 2007-159368A, there is disclosed a control for suppressing the torque discontinuity at the time of control mode switching. Specifically, in the voltage phase control, a torque or an electric power is estimated, and the estimated value is fed back to the voltage phase command value. Similarly, in the current control, a torque or the like is estimated, and the estimated value is fed back to the torque command value. As a result, a steady-state torque is maintained constantly regardless of the control mode.